In recent years, JPEG, MPEG and the like have been widely used as high-performance image compressive coding methods in the fields of broadcasting, communication, and storage. In the JPEG coding method, an image is coded by removing redundancy in the space direction from the image. In the MPEG coding method, an image is coded by removing redundancies in the space direction and the time direction from the image.
Usually, when the bit rate is high (when the compression ratio is small), degradation in image quality is inconspicuous in the JPEG and MPEG coding methods. However, when the bit rate is low (when the compression ratio is large), degradation in image quality, i.e., coding noise, becomes conspicuous. There are blocking artifact (also referred to as block noise or block distortion) and ringing artifact (also referred to as mosquito noise or corona noise) as typical coding noises in the JPEG and MPEG coding methods.
The blocking artifact is a phenomenon in which the edges of a block sharply look like a tile. This phenomenon is caused by an image signal in a block that has only low frequency components, and the values of the frequency components differ from those of adjacent blocks.
The ringing artifact is a phenomenon in which flicker occurs in the vicinity of the edges of a block as if mosquitoes are flying. This phenomenon is caused by high frequency components possessed by an image signal that are lost in the quantization process.
The blocking artifact and the ringing artifact are very conspicuous as degradation in image quality, in contrast with the conventional analog noise. A method for removing these noises is described in, for example, Japanese Patent No. 2643636. Recently, the method of removing noise in the time direction has been widely used.
However, when an image is subjected to noise removal according to the above-mentioned conventional method, the viewer cannot easily judge how much the image quality is improved as compared with the original image or which part of the image is subjected to noise removal, from the image after noise removal.